How long is the wire?
How long is the connection between your sound card and the bus?
How long is your bus?
What clock speed is the bus that contains your sound card running at?
What sound API are you using?
etc etc.

ASIO is theoretically capable of getting extremely low (in the 1-5ms range, if I remember right) latencies, but it requires a good system setup. If you want REALLY low latency, just plug the mic into your speakers or into a simple amplifier. Other than this, though, it’s impossible to get anything resembling an accurate number without doing any tests. There are just too many factors involved.